DC Motor run at much lower voltage???
adm...@c0.com
I'd like to run it:
a) Much slower / lower power to pump water.
b) Directly off a single 100W / 17 V solar panel.
Any advice on if I may smoke the panel or motor, or some other issue?
I don't think this is a problem, but would like any feedback first.
It's less important that it be all that efficient than it be very
simple, and just run when it's sunny.
Thanks,
Mike
Sylvan Butler
>I have a 1/2 Hp DC PM Motor, 90V, 5 Amps.
>
>I'd like to run it:
>a) Much slower / lower power to pump water.
>b) Directly off a single 100W / 17 V solar panel.
Hmm. I think you are going to need a PWM style motor controller. Usually you
can run a DC PM motor on 1/4 to 1/2 of its nominal voltage rating, but 17v for
a 90v motor is really pushing it! Or is that 90v the MAXIMUM voltage rating?
Furthermore, 1/2HP is about 375watts at BEST, and 5a*90v is 450watts. Usually
when you run a motor on lower voltage, the current goes up so the wattage
doesn't drop as much as expected, but it creates more heat and the motor runs
slower so less air circulation. Kind of invites overheating, but since
normally the motor cannot run "too slow" it all works out.
Your power level is most likely too low to do anything. Probably the full
100w could be dissappated in the motor without turning it or harming it.
sdb
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Gerry Schneider
adm...@c0.comm wrote:
> I have a 1/2 Hp DC PM Motor, 90V, 5 Amps.
>
> I'd like to run it:
> a) Much slower / lower power to pump water.
> b) Directly off a single 100W / 17 V solar panel.
>
> Any advice on if I may smoke the panel or motor, or some other issue?
> I don't think this is a problem, but would like any feedback first.
>
> It's less important that it be all that efficient than it be very
> simple, and just run when it's sunny.
>
Actually, you're kind of in luck in that the PV panel can put out a fair
amount of current, enough to torque the motor under stall conditions.
This means you won't need a power matching type of switcher (called an
MPPT), but can just connect directly. Your motor should start to spin OK
with 5 amps, but won't go very fast because of the limited voltage. I've
got a lot of 90VDC and 100VDC motors of that power range, both PM DC like
yours and series wound, and the PM types are easiest to spin slowly (I
use mostly 36VDC). The main thing to watch out for is the chance of
overheating in case the motor has built-in fan cooling, since at low RPM
there won't be much air flow; you can test it out by jamming the shaft
with vise grips and applying full PV current. This won't hurt the panel
and you'll feel the motor heating up before it gets to a dangerous
temperature. If you can still comfortably touch the case after 10 minutes
there shouldn't be a problem. As to how much power you'll get from a 14V
panel (17V is open circuit, right?), that's hard to tell. An MPPT would
raise the available voltage or available current as needed to always
maximize the power, but that's an expensive approach. The Linear Current
Booster version of the MPPT (sold through Homepower magazine ads?) is
cheaper but won't raise the voltage, which is mainly what you need. It
all depends on the load characteristics of the pump.
Sorry to ramble; the short answer is that it's not going to smoke the
panel (they can take a short circuit no problem) and the motor won't burn
out at only a few amps, so go for it.
Good luck,
Gerry
Duane C. Johnson
Mike <adm...@c0.com> wrote:
> I have a 1/2 Hp DC PM Motor, 90V, 5 Amps.
Here are some general rules of thumb for DC motors.
1. The output torque is proportional to the current.
There is some small current that is needed to
overcome the residual mechanical bearing and
commentator friction.
2. For a given torque or current the RPM is
directly proportional to the applied voltage.
3. DC motors are about 85% efficient. Maybe more.
4. DC motors run over a very wide RPM range.
From 0 RPM or at least very low RPM on up to
the nameplate rating.
> I'd like to run it:
> a) Much slower / lower power to pump water.
> b) Directly off a single 100W / 17 V solar panel.
Ok.
> Any advice on if I may smoke the panel or motor,
> or some other issue? I don't think this is a
> problem, but would like any feedback first.
You won't harm the motor nor the panel.
> It's less important that it be all that efficient
> than it be very simple, and just run when it's sunny.
Do an experiment.
Connect your motor to the panel.
Connect an ammeter in series and a voltmeter in parallel.
Then do some measurements.
Apply a breaking load to the motor. Record the RPM vs Current.
Pumping water requires a certain amount of torque to
overcome the force of gravity. You should be able to find
a gear reduction that will run your pump.
> Thanks,
> Mike
Have fun! Duane
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Bloody Viking
adm...@c0.comm (adm...@c0.com) wrote:
: I have a 1/2 Hp DC PM Motor, 90V, 5 Amps.
: I'd like to run it:
: a) Much slower / lower power to pump water.
: b) Directly off a single 100W / 17 V solar panel.
The engine might run on your panel, but it will have very little power. The
resistance/impedance of the engine will be fairly constant, so dropping
voltage by half reduces amps drain by half and power to only 1/4 of the
original power.
I'm familiar with the series-wound engines as those are my favourite type, but
the calculations for them will work out for the permanent magnet type too.
With the specified voltages and the amps for one, the amps drain at the 17
volts will be an amp or so. So, the engine will turn most likely but be only
17 watts. At that power, it will almost certainly not be able to pump much
water at all.
While a permanent magnet engine has the best theoretical efficiency (after
all, the field is there, avoiding power drain to make it) the series-wound
engine will always rev up until its horsepower matches the mechanical load. In
fact, it's dangerous to have one without a load at rated voltage; the rotor
could blow up as it revs up to where the bearings impose the load! That's why
series-wound engines are _always_ direct-coupled to its load. This automatic
load-matching is what makes series-wound engines so cool. The starter of your
car is this type and the exemption to the direct-couple rule too.
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Bloody Viking
Sylvan Butler (Zsylvan+...@Zcyberhighway.Znet.invalid) wrote:
: Hmm. I think you are going to need a PWM style motor controller. Usually you
: can run a DC PM motor on 1/4 to 1/2 of its nominal voltage rating, but 17v for
: a 90v motor is really pushing it! Or is that 90v the MAXIMUM voltage rating?
I bet the 90 volts is its nominal rating. If so, it won't be too useful at 17
volts. It would be like the difference between 120 volts and 24 volts with a
series-wound engine driven fan. I have this one 120 volt series-wound engine
that takes 100 watts. Put a 5-inch fan blades on it and it's a mini hurricane.
Loud as fuck, it'll blow papers all over the place and create quite a wind
gust. At 24 volts, it's output is real feeble, being only 1/25 of the rated
power. (not counting rolling friction of the bearings, which reduces power to
even less)
As far as overvoltage, some model aeroplane hobbyists like to use series-wound
engines in all-electric planes. They'll find one for a given voltage and
double it. Since it will drain the batteries real fast, it'll overheat
somewhat but is not a problem as the battery is drained before the engine is
damaged. That would be like taking my series-wound mini hurricane fan and
plugging it into 240VAC. 4 times the power, it would create a massive wind
gust. The 100 watt "hurricane fan" I own does warm up the engine, but not
excessively. It's just warm to the tough, not hot. But its air output is
plenty comparable to a normal fan yet you can hold it in your hand. Not to
mention, series-wound engines run on ANY power! Dirty or clean, they don't
care.
Bloody Viking
Duane C. Johnson (red...@redrok.com) wrote:
: Here are some general rules of thumb for DC motors.
: 1. The output torque is proportional to the current.
: There is some small current that is needed to
: overcome the residual mechanical bearing and
: commentator friction.
This small current ends up being the item that throws off calculations
compared to real world. But it's not much.
: 2. For a given torque or current the RPM is
: directly proportional to the applied voltage.
Yep. The corollary is that apart from the above bearings item, the power is in
proportion to the square of the voltage applied. It's about like the engine is
simply a resistor.
: 3. DC motors are about 85% efficient. Maybe more.
My favourire are the series-wound type. What are their efficiency? The
inefficiency comes from the bearings and the resistance of the windings. Is
that figure for permanent magnet types? If so, the series-wound type should be
about 70 percent efficient.
: 4. DC motors run over a very wide RPM range.
: From 0 RPM or at least very low RPM on up to
: the nameplate rating.
Definitely my expierence with DC (and universal) engines.
: You won't harm the motor nor the panel.
In the case of the 90 volt/5 amp engine and the 17 volt 100 watt panel,
there's no danger of fucking up either item. Instead the problem is that the
work output of the engine will be too feeble to do anything useful. The engine
will be draining only 17 watts or so, so he needs to develop a voltage doubler
to get a better match and have more power for his pump. If he had a voltage
doubler device, the power will be squared but within the panel's power output.
The drain will be 68 watts with a perfect voltage doubler. That'll leave some
power to fill a battery pack for another use.
: Pumping water requires a certain amount of torque to
: overcome the force of gravity. You should be able to find
: a gear reduction that will run your pump.
Chances are he will need speed-increasing gearing to run a pump. Certainly for
a centrifugal pump.
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